The present invention pertains to apparatus for controlling, manipulating and tailoring the shape of optical pulses.
An optical pulse shaping system capable of producing output pulses of a desired temporal and spectral profile has application in areas such as optical digital communications, quantum optics and laser fusion. For example, laser fusion experiments require the production of optical pulses on the picosecond to nanosecond time scale with properly shaped temporal intensity profiles. High speed optical communications systems also require pulse shaping, forming and coding.
Several approaches to pulse shaping have been proposed in the prior art and these generally use either active or passive shaping techniques. Active pulse shaping techniques include electrooptic deflectors and Pockels cells. Passive pulse-shaping techniques include mirror stackers, etalon stackers, intensity dependent filters, flat lens shapers, nonlinear interferometers, birefringent filters and double-grating pulse shaping systems.
In particular, an article entitled "Optical Pulse Shaping With a Grating Pair" by J. Agostinelli, G. Harvey, T. Stone and C. Gabel in Applied Optics, Vol. 18, No. 14, 15 July 1979, pp. 2500-2504 discloses the concept of a passive pulse-shaping system that uses a pair of diffraction gratings along with various filters of amplitude and/or phase type to alter the temporal and/or instantaneous spectral profile of the input pulse. Amplitude filters will attenuate certain spectral components and therefore certain portions of the temporal profile of the input pulse will be attenuated. Phase filters will shift various groups of spectral components in time.
As shown in the article, an unshaped input pulse enters the system by impinging upon a first diffraction grating. The diffracted beam emerges as a diverging fan of rays due to the bandwidth of the input pulse and the dispersive nature of the grating. The diverging beam then impinges upon a second diffraction grating of identical groove spacing as the first grating. The angles of the two gratings are precisely matched, so that after the second diffraction, the rays emerge parallel to the input ray direction. A mirror is set perpendicular to the beam emerging from the second grating in order to reflect light back through the pair of diffraction gratings, with each ray retracing its steps, so that a collimated beam emerges at the output of the system in the opposite direction of the incident beam.
Each spectral component of the input pulse traverses a different distance in passage through this system. However, due to the negatively dispersive nature of the grating pair, higher frequency components of the input pulse emerge prior to the lower frequency components.
In the plane of the mirror, called the filter plane, there is both spatial and temporal transposition of the spectral components of the input pulse. Amplitude filters are inserted in the "filter plane" to attenuate certain spectral components and therefore attenuate certain portions of the temporal profile of the output pulse. Phase filters are inserted to shift various groups of spectral components in time.
Further, the article discloses the use of various opaque strips placed at various places in the "filter plane" to alter the shape of the output pulse and the use of a plate having a continuously varying transmission function to produce a linearly ramped output pulse.
Unfortunately, the output pulse from the system shown in the article is linearly frequency modulated, "as predicted by linear systems theory (the Fresnel transform of a band-limited Gaussian is a wider Gaussian with linear FM)". Furthermore, the output pulse is not transform-limited, i.e., the output pulse has more spectral width than is necessary to support the features of the shape of the intensity profile, and it is necessarily longer than the input pulse. Since a transform-limited pulse will propagate a greater distance in an optical fiber than a non-transform-limited pulse before being distorted by dispersion of the group velocity, this presents a substantial drawback in using output pulses from the system disclosed in the article in optical digital communications systems.